Sea level rise doesn’t necessarily spell doom for coastal wetlands

Rising sea levels don’t have to spell doom for the world’s coastal wetlands. A new study suggests salt marshes and other wetlands could accumulate soil quickly enough to avoid becoming fully submerged — if humans are willing to give them a little elbow room.

The new study builds on previous work that suggests rising seas will increase sediment buildup in some parts of coastal wetlands. This increased sediment, as well as human adaptations to allow wetlands to move inland as the seas rise, could allow the coastal fringes to not only survive but to increase their global area by as much as 60 percent, researchers report September 13 in Nature.

Humans have good reason to preserve the world’s salt marshes and mangrove forests. These coastal zones, occupying the area from the coastline to the highest inland push of the tides, perform key services including filtering pollutants, pulling and storing carbon dioxide from the atmosphere and protecting communities from storms. Rising ocean waters, however, could drown these ecosystems. Current sea level rise projections suggest 20 percent to 90 percent of the world’s coastal wetlands could disappear, depending on how warm the planet gets and how high the seas rise.

But the picture may not be so dire. Mark Schuerch, a coastal geographer at the University of Lincoln in England, says previous estimates of wetland resilience don’t consider that rising seas inundate marshes more frequently, particularly those at lower elevation. The water carries loads of sediment, so more frequent sediment deliveries may actually help the lower marshes rapidly build up soil elevation even as the water rises. “That made us think that, by including this in global models, we’d get higher resilience of coastal wetlands,” Schuerch says.

The researchers also wanted to investigate the role of coastal communities. Local-scale and even regional-scale studies suggest marshes can survive if allowed to expand inland, Schuerch says. People would need to make way by, for example, moving seawalls, rerouting coastal roads, giving up coastal agriculture or sacrificing seaside real estate.

Schuerch and his team considered global warming scenarios leading to sea level rise from 29 centimeters to 110 centimeters by the year 2100. The researchers also looked at global coastal population data, envisioning what population density numbers would allow wetlands to expand inward naturally — and what population density numbers would require communities to give marshes room to move.

Factoring in sediment supply alone improved the picture somewhat: Even under the highest sea level rise scenario, global wetland losses dropped 30 percent from their current global area of 200,000 square kilometers, rather than 90 percent.

Introducing human adaptations proved even more significant, allowing coastal wetlands not only to survive, but to thrive. Making way for inland marsh migration could increase the total wetland area around the planet by as much as 60 percent, the team found.

That 60 percent number is a long shot, Schuerch acknowledges, as many communities might balk at shifting their infrastructure and agriculture. Ultimately, “it’s going to be a cost-benefit analysis,” he says. “But if there are enough of these nature-based solution projects in enough regions, it’s possible.”

But some scientists suggest it’s not just a long shot, but wholly unrealistic to expect such accommodation, particularly in less-developed regions such as Southeast Asia or South America. “It’s not going to happen,” says Randall Parkinson, a coastal geologist at Florida International University’s Sea Level Solutions Center in Miami Beach. Parkinson says he’s concerned that the paper is more of “an intellectual exercise” that doesn’t include any analysis of the real-world effectiveness of such coastal zone management strategies.

Still, the study gives a good first approximation of what sediment changes and policy shifts might mean for coastal wetlands, says Jonathan Woodruff, a coastal geologist at the University of Massachusetts Amherst who wrote a commentary on the study in the same issue of Nature. “Putting numbers to that is really important,” he says, but he notes that many questions remain. There has been little research on what happens as wetlands migrate inland, for example, and that process may not be so simple.

“There are a lot of things we still don’t have a firm grasp on,” Woodruff says, such as whether vegetation or soil conditions might hinder wetland migration, or how different species of mangroves can promote soil accumulation (SN: 8/18/18, p. 19).

Simulating how tidal wetlands might respond to sea level rise at the global scale is “a Herculean feat,” Woodruff says. “But you’ve got to start somewhere.”